1. Field of the Invention
This invention relates to novel polymers and the use of the novel polymers for the formulation of photoresists particularly suitable for deep U.V. exposure having the capability of forming highly resolved features of submicron dimension.
2. Description of the Prior Art
Photoresists are photosensitive films used for transfer of images to a substrate. They form negative or positive images. After coating a photoresist on a substrate, the coating is exposed through a patterned photomask to a source of activating energy such as ultraviolet light to form a latent image in the photoresist coating. The photomask has areas opaque to activating radiation and other areas transparent to activating radiation. The pattern in the photomask of opaque and transparent areas defines a desired image that may be used to transfer the image to a substrate. A relief image is provided by development of the latent image pattern in the resist coating. The use of photoresists is generally described, for example, by Deforest, Photoresist Materials and Processes, McGraw Hill Book Company, New York (1975), and by Moreau, Semiconductor Lithography, Principles, Practices and Materials, Plenum Press, New York (1988).
Known photoresists can provide features having resolution and size sufficient for many existing commercial applications. However for many other applications, the need exists for new photoresists that can provide highly resolved images of submicron dimension.
Highly useful photoresist compositions are disclosed in U.S. Pat. No. 5,128,232 to Thackeray et al. The patent discloses, inter alia, the use of a resist resin binder that comprises a copolymer of non-aromatic cyclic alcohol units and phenol units. The disclosed photoresists are particularly suitable for exposure to deep U.V. (DUV) radiation. As is recognized by those in the art, DUV radiation refers to exposure of the photoresist to radiation having a wavelength in the range of about 350 nm or less, more typically in the range of about 300 nm or less.
A class of photoresists for which the copolymers of the non-aromatic cyclic alcohol and phenol units are particularly suitable are the compositions that comprise a resin binder, a photoacid or photobase generator and one or more other materials that will result in curing, cross-linking or hardening of the composition upon exposure to activating radiation and heating as necessary to complete the cure. A preferred composition is an acid-hardening photoresist that comprises a photoacid generator, a resin binder comprising the non-aromatic cyclic alcohol and phenol units, and an amine-based cross-linker such as a melamine-formaldehyde resin exemplified by the Cymel resins available from American Cyanamid. These acid-hardening resists are described in numerous publications including European Patent Applications Nos. 0 164 248 and 0 232 972 and in U.S. Pat. No. 5,128,232, each incorporated herein by reference for their disclosure of conventional DUV activated, acid hardening photoresist compositions and processes for their use.
An important property of a photoresist is image resolution. A developed photoresist image having lines not exceeding 1 micron in width and having vertical side-walls is highly desired to permit transfer of fine line images to an underlying substrate. Numerous factors are involved in the formation of high resolution images. One such factor is the strength of the developer used to develop an imaged photoresist coating. It is desirable to use a strong, high normality developer to rapidly penetrate the photoresist coating during the development process and to rapidly remove residue from the developed image. Consequently, developers are used at a strength that is only somewhat below that strength that would cause attack on portions of the exposed photoresist coating where development is undesired.
In practice, a development problem has been encountered when a strong developer is used to develop the acid hardening photoresists described above, especially when the resin binder used is a copolymer of an aromatic cyclic alcohol and a phenol. The problem involves microbridging between developed features. The term microbridging as used herein means a series of fine tendrils or strands of unremoved photoresist passing between adjacent photoresist features in a developed photoresist coating. Using a tetramethyl ammonium hydroxide (TMAH) developer for purposes of illustration, microbridging is encountered as the developer strength exceeds 0.20 N TMAH. However, for best results, TMAH developers are desirably used at a strength of about 0.26 N TMAH. Thus the problem of microbridging has limited the useable strength of the TMAH developer to about 0.15 N TMAH. This reduction in the strength of the TMAH developer has resulted in a sacrifice in image resolution, clean development and development time.